Make it possible to render without a depth buffer.

Some GL implementations (especially on embedded) will give us an
OpenGL context without a depth buffer. In low memory scenarios, it
might also be feasible to request a non-depth buffer context to save
the depth buffer memory. The renderer deals with this by treating all
nodes as translucent, by not adjusting the shaders and by not creating
the extra z-order vertex attribute for merged nodes.

Change-Id: I8edc92d530daa3e2628df2ba52901b47d87eaf26
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>